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Also just remembered - disconnect the power supply and check for voltage between neutral and ground. Shouldnt have more than 0.5vac if so you may have a neutral/grounding issue. It's put an end to me banging my head against the wall before... on a Trane 90% if I remember correctly. Read between 15-25v while also getting 95-105v hot to ground.

i installed a new system and went to fire off heat. i had a wire that went to safety switch on pump that broke 24v from board. when the unit went to energize gas valve it would drop out. I had 24v the entire time.

Come to find out, the wire i ran to the pump safety switch had a break in the wire. When the gas valve went to energize it would drop everything out.

make sure all your connections are good and no broken wires. check resistance thru your switches.

I too had a similar situation, found poor connection at the molex plug. Hard to find some times, try moving wires around and observing meters and listening while you pray. You will get it. Jump 24v to gas valve and make sure valve is good before investing time in searching for your problem. Good luck.

Sounds like bad gas valve. Temporarily jump out the pressure switch and try a call for heat. Make sure you do it at the right time in the ignition sequence our you will fault on "swtich stuck closed." If no change, try unplugging the gas valve, make a call for heat and listen to the relays on the board. If the gas valve relay clicks and holds for the full four or five second trial for ignition it's most likely a short in the valve.

Went back, all indicators were bad gas valve. Removed old gas valve, before installing new valve I hooked up low voltage to it. It worked correctly, so I installed. Once installed, fired system and gas valve lost 24v in less than a second...still no error code.

Continued to troubleshoot, was preparing to try a different board when I suddenly got an open limit switch error. Traced to rollout switch...it was open, it hadn't been before. Replaced with new rollout, fired furnace...fired correctly.

Furnace ran for about 3 min, then tripped the new rollout. Checked temp in burner box, not sealed...upwards of 200 degrees...opened up coil...dime size holes in 3 cells.

The kicker was that the original rollout for some reason was allowing enough voltage to get by for the furnace to light, yet as soon as there was flame it would trip, but just fast enough to cause the board to remove voltage from gv, never giving a limit fault.

Yes, sadly I assumed the heat x had been checked during pm, but not the case. I had not jumped out that limit, my fault there. I think an existing problem was aggravated when the pm was performed, because the bad limit had to be removed in order to remove the burners. Good learning experience though, both troubleshooting and learning to not assume the previous tech did everything correctly (check heat x).

Yes, sadly I assumed the heat x had been checked during pm, but not the case. I had not jumped out that limit, my fault there. I think an existing problem was aggravated when the pm was performed, because the bad limit had to be removed in order to remove the burners. Good learning experience though, both troubleshooting and learning to not assume the previous tech did everything correctly (check heat x).

Yup assuming has left me scratching my head a time or two... and it always sucks explaining something like that to a homeowner, when your guy was there to assure operation and safety... and now they need a new furnace... hope that all goes well for you guys. I would say that if this were to happen at my company, the new install would be discounted for the trouble... hopefully you have an understanding client! Good luck!

Wow. Here I was getting all cockey and raising my hand like Arnold Horshack (if you don't remember him, p*ss off )

I'm a little intrigued by the "2nd transformer" trick in a puzzler like this to quickly and decisively rule out the gas valve. Though I would just use 36" jumpers off the board xfmr after removing R to prevent a cycle. If you hook up your manometer to GV manifold tap, jump the valve and watch the pressure for a five seconds or so; it should quickly confirm the condition of the valve itself, right??? You could keep the valve open as long as you dare since the system would not be cycling and flame rectification would not shut it down.

In cases where the voltage drops so quick your meter doesn't register - like pressure switches that drop in and out so fast they don't even cause an error.

I caught a rudd/rheem late last year, going behind a 30 year tech. Sideways in an attic crawl. GV was chattering and I can't remember why exactly, but I was having trouble deciding if it was the valve or if control voltage was dropping out. I believe in the end I decided that control voltage was on the valve long and steady enough for it to prove flame, so I went with the valve, put it on later in the day and tested fine. But I recall having misgivings until it was resolved.

So anyone else simply jump control voltage to the GV and watch manifold pressure as a quick and easy test? Thoughts?

We actually did that on this one. Took 24 v from board to valve, fired gv and ran without issue. Allowed us to rule out any issue w gas valve. First time I have done this, it also allowed us to run furnace long enough to trip roll out for good, leading to the root issue.